Atom

For other uses, see Atomic.

Atom
Helium atom ground state
A semi-accurate depiction of the helium atom. The darkness of the electron cloud corresponds to the line-of-sight integral over the probability function of the 1s electron orbital. The magnified nucleus is schematic, showing protons in pink and neutrons in purple. In reality, the nucleus (and the wavefunction of each of the nucleons) is also spherically symmetric. (For more complex nuclei this is not the case.)
Classification
Smallest recognised division of a chemical element
Properties
Mass: ≈ 1.66 × 10−27 to 4.52 × 10−25 kg Electric charge: zero Diameter (approx.): 100 pm to 400 pm

In an atom (Greek άτομον meaning "indivisible") is the smallest possible particle of a chemical element that retains its chemical properties. The word atom originally meant the smallest possible indivisible particle, but after the term came to have a specific meaning in science, atoms were found to be divisible and composed of smaller subatomic particles.

Most atoms are composed of three types of subatomic particles which govern their external properties:

• electrons, which have a negative charge and are the least massive of the three;
• protons, which have a positive charge and are about 1836 times more massive than electrons; and
• neutrons, which have no charge and are about 1839 times more massive than electrons.

Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called nucleons. The electrons form the much larger electron cloud surrounding the nucleus.

Atoms differ in the number of each of the subatomic particles they contain. The number of protons in an atom (called the atomic number) determines the element of the atom. Within a single element, the number of neutrons may also vary, determining the isotope of that element. The number of protons and neutrons in the atomic nucleus may also change, via nuclear fusion, nuclear fission or radioactive decay. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons.

Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called ions. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism atoms are able to bond into molecules and other types of chemical compounds like ionic crystals. Molecules are the smallest division of a general collection of matter, including chemical compounds, which retains chemical properties. Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom. A few types of molecules (for example gas molecules of elements that do not form compounds, such as helium) are composed of only a single atom.

Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions.

History of atomic theory

Main article: atomic theory

Properties of the atom in present theory

Subatomic particles

Main article: subatomic particles

Although the name "atom" was applied at a time when atoms were thought to be indivisible, it is now known that the atom can be broken down into a number of smaller components. The first of these to be discovered was the negatively charged electron, which is easily ejected from atoms during ionization. The electrons orbit a small, dense body containing all of the positive charge in the atom, called the atomic nucleus. This nucleus is itself made up of nucleons: positively charged protons and chargeless neutrons.

Before 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of still smaller particles called quarks. In addition, the electron is known to have a nearly massless neutral partner called a neutrino. Together, the electron and neutrino are both leptons.

Ordinary atoms are composed only of quarks and leptons of the first generation. The proton is composed of two up quarks and one down quark, whereas the neutron is composed of one up quark and two down quarks. Although they do not occur in ordinary matter, two other heavier generations of quarks and leptons may be generated in high-energy collisions.

The subatomic force carrying particles (called gauge bosons) are also important to atoms. Electrons are bound to the nucleus by photons carrying the electromagnetic force. Protons and neutrons are bound together in the nucleus by gluons carrying the strong nuclear force.

Electron configuration

Main article: electron configuration

The chemical behavior of atoms is due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. These configurations are determined by the quantum mechanics of electrons in the electric potential of the atom; the principal quantum number determines particular electron shells with distinct energy levels. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.

The atomic orbital wavefunctions of a hydrogen atom. The principal quantum number is at the right of each row and the azimuthal quantum number is denoted by letter at top of each column.

An electron shell can hold up to 2n² electrons, where n is the principal quantum number of the shell. The occupied shell of greatest n is the valence shell, even if it only has one electron. In the most stable ground state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell. An excited atom's electrons will spontaneously fall into lower levels, emitting excess energy as a photons, until it returns to the ground state.

In addition to its principal quantum number n, an electron is distinguished by three other quantum numbers: the azimuthal quantum number l (describing the orbital angular momentum of the electron), the magnetic quantum number m (describing the direction of the angular momentum vector), and the spin quantum number s (describing the direction of the electron's intrinsic angular momentum). Electrons with varying l and m have distinctive shapes denoted by spectroscopic notation. In the illustration, the letters s, p, d and f (corresponding to l = 0, 1, 2, 3) describe the shape of the atomic orbital. In most atoms, orbitals of differing l are not exactly degenerate but separated into a fine structure. Orbitals of differing m are degenerate but may be separated by applying a magnetic field, creating the Zeeman effect. Electrons with differing s have very slight energy differences called hyperfine splitting.

Nucleon properties

The constituent protons and neutrons of the atomic nucleus are collectively called nucleons. The nucleons are held together in the nucleus by the strong nuclear force.

Nuclei can undergo transformations that affect the number of protons and neutrons they contain, a process called radioactive decay. When nuclei transformations take place spontaneously, this process is called radioactivity. Radioactive transformations proceed by a wide variety of modes, but the most common are alpha decay (emission of a helium nucleus) and beta decay (emission of an electron). Decays involving electrons or positrons are due to the weak nuclear interaction.

In addition, like the electrons of the atom, the nucleons of nuclei may be pushed into excited states of higher energy. However, these transitions typically require thousands of times more energy than electron excitations. When an excited nucleus emits a photon to return to the ground state, the photon has very high energy and is called a gamma ray.

Nuclear transformations also take place in nuclear reactions. In nuclear fusion, two light nuclei come together and merge into a single heavier nucleus. In nuclear fission, a single large nucleus is divided into two or more smaller nuclei.

Atom size and speed

Atoms are much smaller than the wavelengths of light that human vision can detect, so atoms cannot be seen in any kind of optical microscope. However, there are ways of detecting the positions of atoms on the surface of a solid or a thin film so as to obtain images. These include: electron microscopes (such as in scanning tunneling microscopy (STM)), atomic force microscopy (AFM), nuclear magnetic resonance (NMR) and x-ray microscopy.

Since the electron cloud does not have a sharp cutoff, the size of an atom is not easily defined. For atoms that can form solid crystal lattices, the distance between the centers of adjacent atoms can be easily determined by x-ray diffraction, giving an estimate of the atoms' size. For any atom, one might use the radius at which the electrons of the valence shell are most likely to be found. As an example, the size of a hydrogen atom is estimated to be approximately 1.0586×10⁻¹⁰ m (twice the Bohr radius). Compare this to the size of the proton (the only particle in the nucleus of the hydrogen atom), which is approximately 10⁻¹⁵ m. So the ratio of the size of the hydrogen atom to its nucleus is about 100,000:1. If an atom were the size of a stadium, the nucleus would be the size of a marble. Nearly all the mass of an atom is in its nucleus, yet almost all the space in an atom is filled by its electrons.

Atoms of different elements do vary in size, but the sizes do not scale linearly with the mass of the atom. Their sizes (diameters) are roughly the same to within a factor of about 3. The reason for this is that heavy elements have large positive charge on their nuclei, which strongly attract the electrons to the center of the atom. This contracts the size of the electron shells, so that more electrons fit into only a slightly greater volume.

The temperature of a collection of atoms is a measure of the average energy of motion of those atoms above the minimum zero-point energy demanded by quantum mechanics; at 0 kelvins (absolute zero) atoms would have no extra energy above the minimum. As the temperature of the system is increased, the kinetic energy of the particles in the system is increased, and their speed of motion increases. At room temperature, atoms making up gases in the air move at a speed of 500 m/s (about 1100 mph or 1800 km/h).

Elements, isotopes and ions

Atoms are generally classified by their atomic number Z, which corresponds to the number of protons in the atom. The atomic number determines which chemical element the atom is. For example, carbon atoms are atoms containing six protons. All atoms with the same atomic number share a wide variety of physical properties and exhibit the same chemical properties. The elements may be sorted according to the periodic table in order of increasing atomic number.

The atomic mass A, atomic mass number, or nucleon number of an element is the total number of protons and neutrons in an atom of that element, so-called because each proton and neutron has a mass of about 1 amu. The number of neutrons A−Z in an atom has no effect on which element it is. Each element can have numerous kinds of atoms with the same number of protons and electrons but varying numbers of neutrons. Each has the same atomic number but a different mass number. These are called the isotopes of an element. When writing the name of an isotope, the element name is followed by the mass number. For example, carbon-14 contains 6 protons and 8 neutrons in each atom, for a total mass number of 14.

The atomic mass listed for each element in the periodic table is an average of the isotope masses found in nature, weighted by their abundance.

The simplest atom is the hydrogen isotope protium, which has atomic number 1 and atomic mass number 1; it consists of one proton and one electron. The hydrogen isotope which also contains one neutron so is called deuterium or hydrogen-2; the hydrogen isotope with two neutrons is called tritium or hydrogen-3. Tritium is an unstable isotope which decays through a process called radioactivity. Almost all isotopes of each element are radioactive; only a few are stable. The elements with atomic number 84 (polonium) and heavier have no stable isotopes and are all radioactive.

Virtually all elements heavier than hydrogen and helium were created through stellar nucleosynthesis and supernova nucleosynthesis. Most of the elements lighter than uranium (Z=92) have stable-enough isotopes to occur naturally on Earth. (One notable exception is technetium Z=43; although some technetium has been found on Earth, this occurred only after the element was first synthesized artificially.) Several elements that do not occur on Earth have been found to be present in stars. Elements not normally found in nature have been artificially created by nuclear bombardment; as of 2006, elements have been created through atomic number 116 (given the temporary name ununhexium). These ultra-heavy elements are generally highly unstable and decay quickly.

Atoms that have either lost or gained electrons are called atomic ions (with either positive(+) or negative charge(−), respectively).

Valence and bonding

Main article: [[:valence electrons and chemical bond]]

The number of electrons in an atom's outermost shell (the valence shell) governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the columns of the periodic table of the elements. Alkali metals contain one electron on their outer shell; alkaline earth metals, two electrons; halogens, seven electrons; and various others.

Every atom is most stable with a full valence shell. This means that atoms with full valence shells (the noble gases) are very unreactive. Conversely, atoms with few electrons in their valence shell are more reactive. Alkali metals are therefore very reactive, with caesium, rubidium, and francium being the most reactive of all metals. Also, atoms that need only few electrons (such as the halogens) to fill their valence shells are reactive. Fluorine is the most reactive of all elements.

Atoms may fill their valence shells by chemical bonding. This can be achieved one of two ways: an atom can either share electrons with other atoms (a covalent bond), or it can remove electrons from (or donate electrons to) other atoms (an ionic bond). The formation of a bond causes a strong attraction between two atoms, creating molecules or ionic compounds. Many other types of bonds exist, including:

• polar covalent bonds;
• coordinate covalent bonds;
• metallic bonds;
• hydrogen bonds; and
• van der Waals bonds.

Atomic spectrum

Main article: Atomic spectroscopy

Since each element in the periodic table consists of an atom in a unique configuration with different numbers of protons and electrons, each element can also be uniquely described by the energies of its atomic orbitals and the number of electrons within them. Normally, an atom is found in its lowest-energy ground state; states with higher energy are called excited states. An electron may move from a lower-energy orbital to a higher-energy orbital by absorbing a photon with energy equal to the difference between the energies of the two levels. An electron in a higher-energy orbital may drop to a lower-energy orbital by emitting a photon. Since each element has a unique set of energy levels, each creates its own light pattern unique to itself: its own spectral signature.

If a set of atoms is heated (such as in an arc lamp), their electrons will move into excited states. When these atoms fall back toward the ground state, they will produce an emission spectrum. If a set of atoms is illuminated by a continuous spectrum, it will only absorb specific wavelengths (energies) of photon that correspond to the differences in its energy levels. The resulting pattern of gaps is called the absorption spectrum.

In spectroscopic analysis, scientists can use a spectrometer to study the atoms in stars and other distant objects. Due to the distinctive spectral lines that each element produces, they are able to tell the chemical composition of distant planets, stars and nebulae.

Not all parts of the atomic spectrum are in visible light part of the electromagnetic spectrum. For example, the hyperfine transitions (including the important 21 cm line) produce low-energy radio waves. When electrons deep inside large atoms are knocked out (for example by beta radiation), replacement atoms fall deep into the electric potential of the nucleus, producing high-energy x-rays.

Atoms and antimatter

Main article: antimatter

Antimatter can also form atoms, composed of positrons, antiprotons, and antineutrons. Since antimatter is very difficult to produce and store, only a small amount antihydrogen has ever existed on Earth. This was produced at CERN in the ATHENA and ATRAP experiments using the Antiproton Decelerator.

An exotic atom is made from normal matter with an antimatter substitution. The most familiar example is positronium, an analogue to the hydrogen atom in which a positron is substituted for the usual proton nucleus. It is a common phase in the attraction between an electron and positron before the annihilation reaction in which the matter particles are destroyed and two gamma rays are emitted.

Atoms and the Big Bang

In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang almost all atomic material in the universe was created. During this process, nuclei of hydrogen and helium formed abundantly, but almost no elements heavier than lithium. Hydrogen makes up approximately 75% of the atoms in the universe; helium makes up 24%; and all other elements make up just 1%. However, although nuclei (fully-ionized atoms) were created, neutral atoms themselves could not form in the intense heat.

Big Bang chronology of the atom continues to approximately 379,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K. It was then cool enough to allow the nuclei to capture electrons. This process is called recombination, during which the first neutral atoms took form. Once atoms become neutral, they only absorb photons of a discrete absorption spectrum. This allows most of the photons in the universe to travel unimpeded for billions of years. These photons are still detectable today in the cosmic microwave background.

After Big Bang nucleosynthesis, no heavier elements could be created until the formation of the first stars. These stars fused heavier elements through stellar nucleosynthesis during their lives and through supernova nucleosynthesis as they died. The seeding of the interstellar medium by heavy elements eventually allowed the formation of terrestrial planets like the Earth.

See also

• Atomism
• Basic quantum mechanics
• Chemical bond
• Exotic atom
• Infinite divisibility
• List of particles
• Periodic table
• Radioactive isotope
• Superatom
• Super-heavy atom
• Transuranium element

References

• Kenneth S. Krane, Introductory Nuclear Physics (1987)

External links

• How Atoms Work
• Wikibooks FHSST Physics Atom:The Atom
• Wikibooks Atomic structure
• Science aid - atomic structure A guide to the atom for teens.